In the oil and gas exploration and production industry, bores are drilled from surface to access sub-surface hydrocarbon-bearing formations. The drilled bores are lined with tubing, known as casing or liner, and cement is injected into the annulus between the casing and the surrounding bore wall. Typically, the bore is drilled in sections, and after drilling a section that section is lined with casing. Following cementing of the casing, the next section of bore is drilled. However, as the drill bit utilised to drill the next section must pass through the existing casing, the drill bit will of necessity be of smaller diameter than the drill bit used to drill the previous section. It is often considered desirable to enlarge the bore diameter below a section of casing beyond the drill bit diameter, and this is normally achieved by means of an under-reamer mounted above the drill bit.
Particularly in offshore and deepwater wells, getting the largest casing size possible into the ground is critical to ensure target depth (TD) can be reached with the largest bit size possible, thus maximising production and facilitating access. Under-reaming the pilot bore drilled by the fixed diameter drill bit enables casing sizes to be maximised by providing sufficient open hole clearance to allow the maximum pass through casing size to be selected. Since a newly drilled wellbore can quickly become unstable, for example due to formation creep/swelling, it is also important to set casing as early as possible. Operators are therefore focused upon minimising the time delay between reaching target depth (TD) and setting casing.
When a bore section has been drilled and under-reamed it is necessary to circulate the wellbore clean, that is circulate a fluid such as drilling mud or brine in the bore to remove drill cuttings and to ensure the casing is not obstructed when run in hole. High circulation flow rates are often utilised to speed up the cleaning process. Also, as the drill string is pulled from the hole, the bottom hole assembly (BHA) will be rotated to stir up cutting beds for circulation of the cuttings to surface. When a hydraulically activated under-reamer is present in the BHA it is often the case that the under-reamer cutters will extend into the hole opening position when high circulation rates are used. This can result in further cuttings generation (as the BHA is effectively back-reamed up through the wellbore) and additional hole cleaning time.
There have been a number proposals for under-reamers in which it is possible to lock the under-reamer in the retracted (pilot size) configuration when a section has been drilled and under-reamed to minimise the time required to pull out of hole and subsequently run casing. An example of such an arrangement is described in applicant's International patent application, Publication No. WO2007/017651 A1.